Field of the Invention
The present invention relates to a grid-connected system and a grid-connected method, and especially relates to an inverter grid-connected system and a method for implementing three-phase alternating current grid-connected transition.
Description of the Related Art
FIG. 1 shows a block diagram of the related art photovoltaic inverter grid-connected system. A photovoltaic inverter grid-connected system 1 is arranged between a direct current voltage source Vin and an alternating current electric grid 2. The photovoltaic inverter grid-connected system 1 converts a direct current electric power provided by the direct current voltage source Vin into an alternating current electric power and then feeds the alternating current electric power to the alternating current electric grid 2. The photovoltaic inverter grid-connected system 1 comprises a boost module 10, an inverter module 12, a filter 14 and a switching module 16. The boost module 10 is electrically connected to the direct current voltage source Vin. The inverter module 12 is electrically connected to the boost module 10. The filter 14 is electrically connected to the inverter module 12. The switching module 16 is coupled between the filter 14 and the alternating current electric grid 2.
The switching module 16 comprises a first switch S1, a second switch S2, a third switch S3, a fourth switch S4, a fifth switch S5 and a sixth switch S6. The first switch S1 and the fifth switch S5 are connected in series between the filter 14 and the alternating current electric grid 2. The second switch S2 and the third switch S3 are connected in series between the filter 14 and the alternating current electric grid 2. The sixth switch S6 and the fourth switch S4 are connected in series between the filter 14 and the alternating current electric grid 2. Moreover, the switching module 16 is composed of three double pole double throw relays. A relay 160 is composed of the first switch S1 and the second switch S2. Another relay 162 is composed of the third switch S3 and the fourth switch S4. Still another relay 164 is composed of the fifth switch S5 and the sixth switch S6. Two switches (S1 and S2, or S3 and S4, or S5 and S6) in each of the relays (160, or 162, or 164) will be turned on or off at the same time.
FIG. 2 shows a flow chart of the connecting method for the related art switching module. In practical use, when the photovoltaic inverter grid-connected system 1 is connected to the alternating current electric grid 2, the connecting method for the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5 and the sixth switch S6 comprises following steps in order. Step S101: The first switch S1 and the second switch S2 are turned on (namely, the first switch S1 and the second switch S2 are in the closed circuit status). Step S103: The first switch S1 and the second switch S2 are turned off (namely, the first switch S1 and the second switch S2 are in the open circuit status), and the third switch S3 and the fourth switch S4 are turned on. Step S105: The third switch S3 and the fourth switch S4 are turned off, and the fifth switch S5 and the sixth switch S6 are turned on. Step S106: The fifth switch S5 and the sixth switch S6 are turned off. Step S107: Determines whether the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5 and the sixth switch S6 are abnormal or not. Step S109: After step S107, if the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5 and the sixth switch S6 are normal, then the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5 and the sixth switch S6 are turned on at the same time and the photovoltaic inverter grid-connected system 1 proceeds to connect to the alternating current electric grid 2. Step S110: After step S107, if one of the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5 and the sixth switch S6 is abnormal, then the photovoltaic inverter grid-connected system 1 does not connect to the alternating current electric grid 2.
FIG. 3 shows a current waveform diagram of the related art photovoltaic inverter grid-connected system connecting to the alternating current electric grid and shows a sequence diagram of turning-on or turning-off these switches S1˜S6. A high level signal indicates that at least one of these switches S1˜S6 are tuned on. A low level signal indicates that at least one of these switches S1˜S6 are tuned off.
At the timing t1, the first switch S1 and the second switch S2 are turned on. At the timing t2, the first switch S1 and the second switch S2 are turned off, and the third switch S3 and the fourth switch S4 are turned on. At the timing t3, the third switch S3 and the fourth switch S4 are turned off, and the fifth switch S5 and the sixth switch S6 are turned on. At the timing t4, the fifth switch S5 and the sixth switch S6 are turned off. At the timing t5, the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch S5 and the sixth switch S6 are turned on at the same time.
The photovoltaic inverter grid-connected system 1 mentioned above firstly detects whether the switching functions of these switches S1˜S6 are normal or not. If the switching functions of these switches S1˜S6 are normal, the switches S1˜S6 are turned on at the same time. Therefore, the operation is simple. However, at the moment when these switches S1˜S6 are turned on at the same time, a huge inrush current will be generated, as shown in FIG. 3. This huge inrush current will shorten the lives of these switches S1˜S6. Moreover, at least one of the switches S1˜S6 will be damaged (for example, the first switch S1 is damaged as show in FIG. 3) when the photovoltaic inverter grid-connected system 1 is connected to the alternating current electric grid 2. Therefore, the photovoltaic inverter grid-connected system 1 cannot be connected to the alternating current electric grid 2.